The M3 seats are specific. The convertible front seats are similar but still different. There are no differences in M3 door panels and coupe non-M3 panels. That circle cap thing was on cars 1988 and older. Cars 1989 and up did not have it.

It is the location and the length of the handle. E30 M3 specific handles stick out about 3" or so. There are minor differences in door panels, I do not know if it is just due to different production years or m3 specific, but it is barely noticeable. Obviously the m3 seats are harder to find and more expensive, but they are worth it for resale value. If using the car then regular sport seats will work find. I have complete e30 m3 seats and other stuff for sale here and on s14.